The present invention is generally related to stripline filters and more particularly to an improved stripline filter with combline resonators having a transmission zero above the passband.
In the field of portable communication equipment, size and cost are of great concern. Also in many applications, insertion loss must be minimized while maximizing out-of-band attenuation.
Surface acoustic wave (SAW) filters have been used to fill this need because of their small size and good out-of-band attenuation. However, the SAW filter generally requires external matching circuitry eliminating its size advantage, insertion loss tends to be higher than other filters, and the cost is higher than stripline filters.
The coaxial resonator filter will have low passband loss and good attenuation, but will generally be too large to use in portable applications where low loss is not absolutely critical.
Microstrip transmission line filters such as that described in U.S. Pat. No. 4,551,696 have been used in portable applications providing adequate performance at low cost. However a microstrip filter requires more board area than a stripline filter, may have cover detuning problems, and is subject to possible radiation problems.
The interdigital stripline filter such as those described in U.S. Pat. Nos. 4,157,517, 4,418,324, RE3l,470, and 4,609,892 has been frequently used in portable applications and provides adequate performance at a reasonable size and cost. However, when selectivity requirements in portable applications are stringent, the number of poles in such interdigital stripline filters may become excessive. In such case, the combline stripline filter has the advantage of an additional zero above the passband, so will generally require one less pole. One less pole will mean less insertion loss and less area. A further slight size advantage is gained in that the combline stripline filter will generally have closer resonator spacings for the same bandwidth filter than the interdigital stripline filter. However, combline stripline filter currently available do not realize these advantages due to difficulties in implementing the required capacitances while avoiding undesirable stray capacitive coupling.